Help with Chromotherm III Showing -AC

My thermostat is showing the -AC which is supposed to mean that it is running on temporary battery power. I am not sure why this just started happening. We have not had any power failures and the fan on the furnace unit continues to run and moving the fan control to auto has no affect. The AC unit does not come on even though the setting is well below the current temperature.

I can move the selector switch back and forth between auto and on for the fan control and the -AC goes away temporarily, the system light comes on and then back to the -AC.

Please help. I suspect something at the unit rather than with the thermostat.

TIA

Mike

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Help with Chromotherm III Showing -AC

Try turning off the t-stat (heat/off/cool), and checking that it is properly mounted. Maybe the pin connectors aren't making good contact. Try turning the furnace off at it's switch for 15 or so seconds then on. Sometimes it can reset the furn board. Update us if that doesn't help. Does the furn have a circuit board, and can you see a flashing light?? How many blinks?

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Help with Chromotherm III Showing -AC

Thanks for the reply.

I have tried taking off the t-stat...cleaning what I can and replacing it to no avail. I tried your suggestion of turning off the unit using the switch at the furnace and same behavior. When I turn the switch back on the fan starts up and no control over the AC unit.

I do not see a board with lights...I see a board where the t-stat wires are connected but there are no lights on the board.

Help with Chromotherm III Showing -AC

I am willing to do that but I was unsure if there was anything wrong with the furnace piece before I wasted money on a new t-stat. I was about to call someone out to take a look but would much rather go with the installation of a new t-stat.